Process of manufacturing high-strength cold rolled steel sheets

ABSTRACT

A process of manufacturing high-strength cold rolled steel sheets containing 0.5 to 2.0 mass % silicon includes a pickling step of thermally annealing a steel sheet in a non-oxidizing atmosphere and thereafter pickling the steel sheet to dissolve away 0.5 g/m 2  to less than 2.0 g/m 2  of the steel sheet, and an electroplating step of electroplating the surface of the pickled steel sheet with zinc under such conditions that a coating mass of 100 to 5000 mg/m 2  is obtained.

TECHNICAL FIELD

This disclosure relates to a process of manufacturing high-strength coldrolled steel sheets having excellent conversion treatment properties andpost-painting corrosion resistance.

BACKGROUND

Reducing the weight of car bodies to save CO₂ emissions from vehicleshas recently become a challenge for automobile manufacturers in thefight against global warming. The most effective approach to cutting theweight of car bodies is to reduce the thickness of steel sheets that areused. When a steel sheet is simply reduced in thickness, while itsstrength remains unchanged, the stiffness of the steel sheet isdecreased and the steel sheet becomes incapable of ensuring the safetyof occupants in the event of accidents such as collisions. Thus, therehas been an increasing trend in which car bodies are made usinghigh-strength cold rolled steel sheets which have a reduced thicknessand have been strengthened to make up for the loss in stiffness bythinning of the steel sheets. Most recently, high-strength cold rolledsteel sheets with a tensile strength of 1180 MPa have been increasinglyused for car bodies.

Some effective methods of increasing the strength of steel sheets are toproduce solid solution strengthening or to reduce the size of crystalgrains by addition of alloying elements such as silicon and manganese,to produce precipitation strengthening by the addition ofprecipitate-forming elements such as niobium, titanium and vanadium, andto effect strengthening by formation of a hard transformation structuresuch as a martensite phase.

In general, strengthening by addition of alloying elements results in adecrease in ductility to make it difficult for a steel sheet to bepressed into a shape of a desired part.

Silicon has a smaller effect on ductility deterioration as compared toother elements and is therefore effective in increasing strength whileensuring ductility. Thus, addition of silicon is substantially essentialto produce steel sheets having both good workability and high strength.

However, the equilibrium oxygen partial pressure with oxide of siliconis so low that silicon is readily oxidized even in a reducing atmospherein a continuous annealing furnace used in the general manufacturing ofcold rolled steel sheets. Because of this fact, passage of aSi-containing steel sheet through a continuous annealing furnace causessilicon on the surface of the steel sheet to be selectively oxidized andSiO₂ is formed. When such a steel sheet on the surface of which SiO₂ isformed is subjected to conversion treatment before painting, the SiO₂inhibits the reaction between the conversion treatment liquid and thesteel sheet. As a result, the conversion treatment fails to formconversion crystals under the portions where SiO₂ is present, resultingin the occurrence of non-covered areas on the surface of the steelsheet. A steel sheet having areas that have not been covered byconversion crystals often starts to rust as early as at the stage ofwashing with water just after the conversion treatment. Even if rustdoes not form, a conversion treated steel sheet with non-covered areasexhibits very poor corrosion resistance after painting. Thus, it is verydifficult for Si-containing high-strength cold rolled steel sheets to beused for the manufacture of bodies.

A large number of methods have been proposed to improve the conversiontreatment properties of such Si-containing high-strength cold rolledsteel sheets. For example, Japanese Unexamined Patent ApplicationPublication No. 4-276060 proposes a cold rolled steel sheet in whichoxides having an atomic ratio [Si/Mn] of not more than 1 are formed onthe surface, and a process of manufacturing such steel sheets whichparameterizes the [Si/Mn] ratio in the steel sheet, the annealingtemperature and the ratio of the hydrogen partial pressure to the waterpartial pressure in the atmosphere. However, that process requires thatthe annealing temperature be lowered as the Si content in the steelsheet is increased. When annealing should be performed at a hightemperature to achieve desired strength and ductility, the purpose maybe accomplished by increasing the water content in the atmosphere. Thisapproach, however, results in formation of Fe-based oxide on the surfaceof the steel sheet, thus making the steel sheet unacceptable as aproduct. That is, the technique described in Japanese Unexamined PatentApplication Publication No. 4-276060 cannot be applied to currentmainstream high-strength cold rolled steel sheets containing about 1.0%silicon.

Japanese Patent No. 3934604 proposes a high-strength cold rolled steelsheet containing 0.05 to 2% silicon and satisfies [Si]/[Mn]≦0.4 andwhich is specified in terms of the size and the number per unit area ofSi—Mn complex oxides on the surface of the steel sheet as well as interms of the surface coverage ratio of Si-based oxides on the surface ofthe steel sheet. [Si] and [Mn] indicate the contents of the respectiveelements. Japanese Unexamined Patent Application Publication No.2005-290440 proposes a high-strength cold rolled steel sheet containing0.1 to 1% silicon and satisfies [Si]/[Mn]≦0.4 and which is specified interms of the [Mn/Si] ratio, the size and the number per unit area ofMn—Si complex oxides on the surface of the steel sheet as well as interms of the surface coverage ratio of Si-based oxides on the surface ofthe steel sheet. Further, Japanese Patent No. 3889768 proposes ahigh-strength cold rolled steel sheet containing 0.1 to 2% silicon andsatisfies [Si]/[Mn]≦0.4 and which is specified in terms of the [Mn/Si]ratio, the size and the number per unit area of Mn—Si complex oxides onthe surface of the steel sheet as well as in terms of the surfacecoverage ratio of Si-based oxides on the surface of the steel sheet.Those techniques are applicable to steel sheets containing up to 2%silicon and, as an example of manufacturing the steel sheets discussedabove, it is disclosed that the steel sheets are manufactured whileregulating the conditions for pickling after hot rolling or whilecontrolling the dew point during continuous annealing to −40° C. orbelow. Those methods are based on the premise that the steel sheetssatisfy the [Si]/[Mn] requirement. That is, the methods have a drawbackin that the degree of freedom in the chemical composition of the steelsheet is limited. Controlling the dew point during continuous annealingto not more than −40° C. is very difficult in consideration ofvariations in dew point in actual production lines. Thus, the techniquesdescribed in Japanese Patent No. 3934604, Japanese Unexamined PatentApplication Publication No. 2005-290440, and Japanese Patent No. 3889768are not suited for mass production.

Japanese Unexamined Patent Application Publication No. 2004-323969proposes a cold rolled steel sheet containing not less than 0.4% siliconand satisfies [Si]/[Mn]≧0.4 and which is specified in terms of thesurface coverage ratio of Si-based oxides on the surface of the steelsheet, and a manufacturing process in which such a steel sheet isannealed and thereafter pickled. Japanese Unexamined Patent ApplicationPublication No. 2003-226920 proposes a technique in which a steel sheetcontaining not less than 0.5 mass % silicon is annealed and thereafterthe surface of the steel sheet is ground by at least 2.0 g/m². Further,Japanese Unexamined Patent Application Publication No. 2007-009269proposes a technique in which a steel sheet containing 0.5 to 2.0%silicon is annealed, thereafter treated in an acidic solution at a pH of0 to 4 and a temperature of 10 to 100° C. for 5 to 150 seconds, andfurther treated in an alkaline solution at a pH of 10 to 14 and atemperature of 10 to 100° C. for 2 to 50 seconds. Those techniques areconcerned with the removal of oxide layers formed on the surface afterannealing. For example, the technique of Japanese Unexamined PatentApplication Publication No. 2004-323969 entails the use of an acid witha high concentration to remove Si-based oxides. In that case, such ahigh-concentration acid promotes formation of a passivation film on thebase iron and thus the technique described in that publication does notnecessarily enhance conversion treatment properties. In JapaneseUnexamined Patent Application Publication No. 2003-226920 and JapaneseUnexamined Patent Application Publication No. 2007-009269, further, itis necessary that a grinding section, or sections for the acidicsolution treatment and the alkaline solution treatment be provided inthe production lines. Thus, the techniques described in JapaneseUnexamined Patent Application Publication No. 2003-226920 and JapaneseUnexamined Patent Application Publication No. 2007-009269 are not viabledue to the need for a long and large facility and also due to theincrease in cost.

Japanese Unexamined Patent Application Publication No. 2006-299351proposes a technique in which the surface of a steel sheet is coatedwith a zinc plating layer having a coating mass of 10 to 2000 mg/m² anda specific crystal orientation, thereby satisfying both gallingresistance and conversion treatment properties. That technique is mainlyaimed at improving galling resistance. With regard to conversiontreatment properties, it is indicated that even with such a smallzinc-coating mass, conversion treatment reaction is activated as aresult of formation of microcells between the zinc-coated portions andthe exposed portions of the steel sheet. In a steel sheet having a highSi concentration, however, a major proportion of the surface of thesteel sheet will have been covered with SiO₂. If such oxidized portionsare on exposed portions of the steel sheet, microcells will notnecessarily be formed.

As discussed hereinabove, no sufficient techniques exist which canprevent a decrease in the conversion treatment properties of cold rolledsteel sheets to which silicon has been added for the purpose ofincreasing strength while maintaining ductility. This circumstance hashindered the application of high-strength cold rolled steel sheets toautomobile bodies.

It could therefore be helpful to provide a process of manufacturing coldrolled steel sheets exhibiting excellent conversion treatment propertiesand post-painting corrosion resistance.

SUMMARY

We noted that SiO₂ formed on the surface of a steel sheet inhibitsdissolution of iron in that portion and, consequently, makes itdifficult for conversion crystals to be formed there. We then assumedthat conversion crystals will be formed if the dissolution reaction issomehow allowed to take place. We thus focused our attention on zincphosphate layers that are general conversion layers (layers composed ofconversion crystals) and conducted various studies based on theassumption that the prior application of such thin zinc as serving onlyto help formation of a conversion layer on the surface of a cold rolledsteel sheet will make it possible to form a zinc phosphate layer afterconversion treatment.

As a result, we found that SiO₂ formed on the surface of a steel sheetlocally has a form of films extending over relatively large areas, andzinc cannot be precipitated on such SiO₂ films and thereby fails toprovide an effect of helping formation of a conversion layer. We furtherfound that formation of a conversion layer on a steel sheet having suchSiO₂ on its surface cannot be promoted simply by changing the Zn-coatingmass.

We thus carried out further studies and found that a dense and uniformconversion layer may be formed on any type of a high-Si cold rolledsteel sheet by performing pickling with respect to an annealed steelsheet to remove an amount of 0.5 g/m² or more and thereafterelectrogalvanizing the pickled steel sheet.

We thus provide:

(1) A process of manufacturing high-strength cold rolled steel sheets,the high-strength cold rolled steel sheets containing 0.5 to 2.0 mass %silicon, the process including an annealing and pickling step ofthermally annealing a steel sheet in a non-oxidizing atmosphere andthereafter pickling the steel sheet to dissolve away 0.5 g/m² to lessthan 2.0 g/m² of the steel sheet, and an electroplating step ofelectroplating the surface of the pickled steel sheet with zinc undersuch conditions that a coating mass of 100 to 5000 mg/m² is obtained.

(2) The process of manufacturing high-strength cold rolled steel sheetsdescribed in (1), wherein the non-oxidizing atmosphere is obtained byintroduction of a mixture gas containing nitrogen and hydrogen, thehydrogen content in the non-oxidizing atmosphere is not more than 10 vol%, and the temperature of heating during the thermal annealing is notmore than 900° C.

(3) The process of manufacturing high-strength cold rolled steel sheetsdescribed in any one of (1) and (2), wherein the process furtherincludes an aqueous solution contact step of bringing the steel sheetafter the electroplating step into contact with a P-containing aqueoussolution having a concentration of not less than 0.001 g/L at atemperature of the P-containing aqueous solution of not less than 30° C.

The strength of steel may be increased without causing a decrease inductility. Thus, the workability of high-strength cold rolled steelsheets may be improved. Further, it is possible to form a dense anduniform conversion layer by conversion treatment of steel as the base ofpainting. Consequently, high-strength cold rolled steel sheetssatisfying high strength and workability and also exhibiting excellentconversion treatment properties may be obtained. Further, thehigh-strength cold rolled steel sheets that have been subjected toconversion treatment exhibit excellent corrosion resistance after thehigh-strength cold rolled steel sheets are painted.

DETAILED DESCRIPTION

Our steel sheets and methods will be described in detail hereinbelow.The scope of this disclosure is not limited to the examples describedbelow.

We provide a process of manufacturing high-strength cold rolled steelsheets containing 0.5 to 2.0 mass % silicon. The process includes anannealing and pickling step and an electroplating step.

The annealing and pickling step is a step in which a steel sheet isthermally annealed in a non-oxidizing atmosphere and thereafter 0.5 g/m²to 2.0 g/m² of the surface of the steel sheet is dissolved away bypickling. The electroplating step is a step in which the surface of thepickled steel sheet is electroplated with zinc under such conditionsthat a coating mass of 100 to 5000 mg/m² is obtained.

First, the steel sheet to be thermally annealed will be described. Theunit “%” used for the chemical composition and concentrations indicates“mass %” unless otherwise mentioned.

The steel sheet contains 0.5 to 2.0% silicon. Addition of silicon makesit possible to strengthen steel by solid solution strengthening with arelatively small decrease in formability. Sufficiently high strength maybe obtained by adding silicon to a Si content of 0.5% or more.Controlling the Si content to be 2.0% or less ensures a small decreasein ductility, and the decrease in production efficiency during coldrolling may be prevented.

Elements other than silicon are not particularly limited. It is,however, preferable that the steel sheets contain the following elementsin the following amounts.

The steel sheet preferably contains 0.05 to 0.25% carbon because carbonis an element that contributes to formation of such phases as retainedaustenite, bainite and martensite necessary to strengthen steelmicrostructures. When the need arises to add carbon appropriately toobtain desired microstructures, it is preferable to add carbon so thatthe C content becomes 0.05% or more. If the C content exceeds 0.25%, adecrease in weldability is sometimes caused. It is therefore preferablethat the C content be controlled to not more than 0.25%. The C contentis more preferably 0.05 to 0.10%.

The steel sheet preferably contains 0.5 to 3.0% manganese. Manganese canstrengthen steel by solid solution strengthening and also enhances thehardenability of steel to promote formation of retained austenite,bainite and martensite. When the need arises to add manganeseappropriately to obtain desired microstructures, it is preferable to addmanganese so that the Mn content becomes 0.5% or more. The effects aresaturated after the Mn content exceeds 3.0% and any further additiononly increases the cost. It is therefore preferable that the Mn contentbe controlled to not more than 3.0%. The Mn content is more preferably1.6 to 2.6%.

The steel sheet preferably contains 0.005 to 0.05% phosphorus.Phosphorus is a solid solution strengthening element and is usuallyeffective in obtaining high-strength cold rolled steel sheets. The Pcontent is preferably controlled to 0.005% or more. Any P contentexceeding 0.05% sometimes leads to a decrease in spot weldability. The Pcontent is more preferably 0.02 to 0.03%.

The steel sheet may contain 0.005% or less sulfur. Sulfur isprecipitated as MnS in steel and this precipitate causes a decrease inthe stretch flangeability of steel sheets. The S content is morepreferably not more than 0.0020%.

The steel sheet preferably contains 0.005 to 0.06% aluminum. Aluminum isan element added as a deoxidizer during steelmaking and is effective inseparating nonmetallic inclusions that would cause a decrease in stretchflangeability in the form of slag. It is preferable to add aluminum sothat the Al content becomes 0.005% or more to obtain this effect. Addingmore than 0.06% aluminum results in an increase in cost. The Al contentis more preferably 0.007 to 0.040%.

The balance after deduction of the aforementioned components ispreferably iron and inevitable impurities. Examples of the inevitableimpurities include oxygen and nitrogen. Oxygen and nitrogen are typicalexamples of impurities mixed inevitably during the refining of steel. Inparticular, nitrogen decreases formability of steel sheets and istherefore desirably removed to the smallest content that is possibleduring the steelmaking step. However, removal of nitrogen to a greaterextent than is necessary increases the refining cost. It is thereforepreferable that the N content be reduced to a substantially unharmfullevel, specifically, to 0.01% or less. The N content is more preferably0.0040% or less.

The steel sheets described above may be produced by any methods withoutlimitation. For example, the steel sheets may be produced from moltensteel having the aforementioned chemical composition. More specifically,first, molten steel adjusted to the aforementioned chemical compositionis formed into a slab by continuous casting or ingot making. Next, theslab is hot rolled directly or after cooling and reheating. Next, theresultant hot rolled sheet is cooled, coiled, pickled and cold rolledinto a steel sheet having a desired thickness. The process from the hotrolling to the cold rolling may be performed by usual methods under anyconditions without limitation.

In the annealing and pickling step, the above steel sheet is thermallyannealed in a non-oxidizing atmosphere and thereafter the surface of thesteel sheet is dissolved away by 0.5 g/m² or more by pickling. Theannealing and pickling step will be described below.

The non-oxidizing atmosphere refers to an atmosphere in which iron thatis the main component of the steel sheet does not substantially formoxides. Because the usual annealing step uses an inert gas such asnitrogen, the atmosphere requires no control of the oxygen concentrationitself. However, the use of a gas having a high dew point renders theatmosphere oxidative to iron. Thus, the dew point is to be not more than0° C. On the other hand, the lower limit of the dew point is notparticularly limited. The lower limit is preferably −50° C. because thecontrol of the water content comes to require a special facility whenthe dew point is below −50° C.

In addition to being non-oxidative to iron, the non-oxidizing atmosphereis capable of chemically reducing a thin surface oxidized layer (basedon iron) that has been formed during the steps up to the cold rolling.Therefore, the non-oxidizing atmosphere is preferably nitrogen gascontaining hydrogen. The required proportion of hydrogen is preferably0.1 to 10 vol %. The thin surface oxidized layer may not be chemicallyreduced sufficiently if the hydrogen proportion is less than 0.1 vol %.The effects on the chemical reduction of the surface oxidized layer aresaturated after the proportion exceeds 10 vol %. When the hydrogenconcentration is 0.01 vol % or less, the oxidized layer on the surfacetends to persist and makes Zn-plating difficult unless pickling iseffected to a sufficient extent. In such cases, it is necessary that thepickling loss be increased as compared to that under other conditions.

The dew point of the atmosphere gas is not particularly limited and maybe set in a general range, specifically, −50 to 0° C. The dew point ofthe atmosphere gas may be controlled appropriately while ensuring thatoxidation of iron is suppressed.

Heating in the thermal annealing may be performed by any method andunder any conditions without limitation. The heating temperature ispreferably 900° C. or below. To heat sufficiently the steel sheet byannealing, the heating temperature is preferably 700° C. or above. Theheating temperature is more preferably 800 to 850° C.

The heating time during the thermal annealing (the total of thetemperature-raising time and the holding time after the maximum steelsheet temperature is reached) is not particularly limited. The heatingtime is preferably 4 minutes or less in view of the easy control of thearea ratio of oxide in the form of films described later. The heatingtime is preferably 10 seconds or more to ensure that the steel sheet isheated sufficiently by annealing.

To enhance conversion treatment properties, it is preferable that thearea ratio, as will be described later, of oxide in the form of filmspresent on the surface of the steel sheet after the annealing becontrolled. The above ranges of the heating temperature and the heatingtime facilitate control of the area ratio of oxides on the steel sheetsurface to an acceptable range.

After thermal annealing, the steel sheet is cooled. The cooling rate andcooling end temperature in this cooling are not particularly limited andany general conditions may be adopted. For example, the cooling rate maygenerally be 5 to 150° C./sec and the cooling end temperature maygenerally be 300 to 500° C.

The thermal annealing in the non-oxidizing atmosphere results in aphenomenon in which oxidizable elements of the composition of the steelsheet are concentrated as oxides on the surface of the steel sheet.Typical examples of such oxides include SiO₂, MnO and Si—Mn complexoxides.

Under the portion where these oxides present on the steel sheet surface,the reaction of a conversion treatment liquid to etch the steel sheetand to precipitate conversion crystals is inhibited. As a result, thesteel sheet exhibits poor conversion treatment property, namely, thesurface of the steel sheet locally has non-covered areas in which thereare no conversion crystals. This decrease in conversion treatmentproperties causes a serious problem particularly when the oxides thathave been concentrated on the surface are present in the form of filmsover relatively large areas of the steel sheet.

The conversion treatment properties of the steel sheet are improved bypickling described below that is performed after the thermal annealing.Specifically, pickling which dissolves away the surface of the steelsheet by 0.5 g/m² or more is performed. The conversion treatmentproperties of the surface of the steel sheet are improved as a result ofthe surface of the steel sheet being dissolved away by 0.5 g/m² or more.Satisfactory conversion layers may be formed even on steel sheets onwhich oxides have been concentrated on their surfaces in the form offilms during annealing.

The above effects are probably obtained due to the following mechanism.When a steel sheet containing relatively large amounts of oxidizableelements such as silicon and manganese is annealed, oxides areconcentrated on the surface with certain distributions. Of such oxidesthat have been concentrated on the surface, some are distributed asrelatively small grains and others are distributed in the form ofrelatively large films. During electrogalvanization, no zinc isprecipitated on the oxides distributed as films because the oxidesconcentrated on the surface are generally insulators and such regions donot allow the flow of electricity. In such regions of the steel sheet,no dissolution of iron takes place when the steel sheet is brought intocontact with a conversion treatment liquid and, further, there is littlezinc that has been deposited by electrogalvanization. As a result, it isimpossible to form a conversion layer due to failure of the conversiontreatment liquid to produce the dissolution reaction. By performingpickling to reduce a specific amount of such an annealed sheet, the ironcomponents on the surface of the steel sheet are allowed to undergo thedissolution reaction. Although the oxides concentrated on the surfaceremain without being dissolved, it is probable that the iron componentspresent under the oxide distributed in the form of films arepreferentially dissolved to form crevices. When this steel sheet iselectrogalvanized, zinc is precipitated on portions of the surface ofthe steel sheet free from the oxide in the form of films while underportions covered with the oxide in the form of films, a zinc plating isprecipitated in the crevices formed at the interfaces between the steelsheet and the oxides. When this galvanized steel sheet is subjected toconversion treatment, even the zinc plating deposited in the crevicesimmediately below the oxide in the form of films is dissolved by thetreatment liquid. It is probable that the zinc in the crevices serves asa starting point of the precipitation of conversion crystals to allow auniform and dense conversion layer to be formed.

Pickling after annealing is a conventional practice. For example,pickling after annealing is described also in the patent literaturesdiscussed above. For example, the techniques described in JapanesePatent No. 3934604, Japanese Unexamined Patent Application PublicationNo. 2005-290440, and Japanese Patent No. 3889768 are such thatSi—Mn-based oxides rather than Si-based oxides are formed in largeramounts as the main oxides and the techniques utilize the fact thatthese Si—Mn-based oxides are soluble. Pickling after annealing may beperformed for the purpose of assisting this dissolution. Since thepurpose of pickling in Japanese Patent No. 3934604, Japanese UnexaminedPatent Application Publication No. 2005-290440, and Japanese Patent No.3889768 is as described above and the techniques do not assumedissolution of the surface of the steel sheet, the disclosed pickling isnot associated with the dissolution of the surface of the steel sheet by0.5 g/m² or more.

Japanese Unexamined Patent Application Publication No. 2004-323969,Japanese Unexamined Patent Application Publication No. 2003-226920, andJapanese Unexamined Patent Application Publication No. 2007-009269describe that strong pickling is performed mainly to remove Si oxide. InJapanese Unexamined Patent Application Publication No. 2004-323969 andJapanese Unexamined Patent Application Publication No. 2003-226920,removal of Si oxide requires that the weight loss of the steel sheet bypickling is 2 g/m² or more. Japanese Unexamined Patent ApplicationPublication No. 2007-009269 describes that Si-based oxides are removedby treatment with an acid and an alkali, and that treatment requiresthat the weight loss of the steel sheet by pickling is 2.0 g/m² or more.Further, in Japanese Unexamined Patent Application Publication No.2004-323969, Japanese Unexamined Patent Application Publication No.2003-226920, and Japanese Unexamined Patent Application Publication No.2007-009269, the Si-based oxides on the surface of the steel sheet areremoved. That is, the structure of the steel sheet surface of interestis different from that of our steel sheets. Although Japanese UnexaminedPatent Application Publication No. 2006-299351 describes thatpretreatment with an acid or an alkali is performed prior toelectrogalvanization, that treatment is only aimed at cleaning andactivation. Pickling for the purpose of cleaning or activation is notrequired to dissolve the surface of a steel sheet in a positive manner,and the pickling loss is usually about 0.1 g/m².

As discussed above, our technique that improves conversion treatmentproperties by pickling is unprecedented.

An important feature resides in that pickling is performed to asufficient extent for a different purpose than in conventionaltechniques. The pickling loss should be 0.5 g/m² or more. If thepickling loss is less than 0.5 g/m², crevices are formed only partiallyand the insufficiency of crevices makes it impossible to obtain theaforementioned effects. Pickling loss that is excessively largedeteriorate conversion treatment properties. Further, such excessivepickling is not practical due to the increase in facility size and alsodue to the increase in treatment time. In view of these, the picklingloss is less than 2.0 g/m².

The type of an acidic liquid used for pickling is not particularlylimited. It is preferable to use nitric acid, hydrofluoric acid,hydrochloric acid, sulfuric acid, or the like. Of these, the use ofsulfuric acid is preferable from viewpoints such as operation safety.The acid concentration of the acidic liquid is not particularly limitedand may be determined appropriately in the range of, for example, 5 mass% to 20 mass %.

The pickling method is not particularly limited and any general methodmay be adopted. Electrolytic pickling is a preferred pickling methodfrom the viewpoint of the easiness in controlling pickling loss.Pickling loss may be controlled by, for example, changing theenergization time while the current density is constant or by changingthe current density while the energization time is constant.

After the pickling treatment as described above, the high-strength coldrolled steel sheet is subjected to the following electroplating step,thereby achieving an enhancement in conversion treatment properties. Thesurface of the pickled steel sheet is electroplated with zinc under suchconditions that a coating mass of 100 to 5000 mg/m² is obtained. Thezinc plating deposited on the surface of the steel sheet promotesformation of conversion crystals. It is therefore necessary that zinc bepresent on the steel sheet surface in an amount sufficient to allow adense and uniform conversion layer to be formed. From this viewpoint,the lower limit of the Zn-coating mass is 100 mg/m². Any increase in theZn-coating mass does not cause problems in conversion treatmentproperties. However, increasing the Zn-coating mass only for the purposeof improving the conversion treatment properties of cold rolled steelsheets themselves leads to an increase in cost. Thus, the upper limit ofthe coating mass is 5000 mg/m².

The conditions for the electroplating step are not particularly limitedas long as zinc may be deposited in the above coating mass on the steelsheet surface in the electroplating step.

Electrogalvanization is usually a method in which a steel sheet as acathode, and an insoluble anode are placed in a zinc plating bath filledwith an acidic plating liquid containing a prescribed amount of zincions, and electrolysis is performed while circulating the plating liquidto form a zinc plating on the surface of the steel sheet. As long as thezinc plating may be formed in a desired coating mass on the surface ofthe steel sheet, there is no limitation on the Zn ion concentration inthe plating liquid, the type of the acidic component in the platingbath, the pH and the temperature of the plating bath, the flow rate ofthe plating liquid being circulated, and the current density during theelectrolysis.

The coating mass may be controlled by, for example, changing the currentdensity while the energization time is constant or by changing theenergization time while the current density is constant.

As mentioned hereinabove, a feature of our method resides in that zincis precipitated in the crevices present between the Si oxide in the formof films and the steel sheet. It is also effective to control the ratioof the crevice portions to the total area. The lower limit of theZn-coating mass is such an amount that zinc can cover the entirety ofthe surface of the steel sheet. Non-conductive Si-based oxides arepresent and crevices are formed at the interfaces between the oxides andthe steel sheet. In this case, zinc can be precipitated in the crevicesand, consequently, the zinc deposits collectively cover the entiresurface of the steel sheet. If, however, the Si-based oxides occupy themajor proportion of the surface of the steel sheet and even if crevicesare present at the interfaces between the oxides and the steel sheet, itis difficult for zinc to be precipitated in such gaps in sufficientamounts. Thus, the ratio of the crevice portions is desirably controlledto 40% or less. Measurement of this ratio is difficult. However,assuming that the Zn-coating mass is in the specified range, the ratioof the crevice portions may be determined by analyzing the steel sheetsurface with a technique such as an electron probe microanalyzer (EPMA)to determine the distribution of zinc and calculating the ratio of theZn-free areas relative to the total surface. This area ratio may becontrolled by controlling the area ratio of the Si-based oxide in theform of films present on the surface of the steel sheet after theannealing.

The portion where SiO₂ films are formed on the surface are insulatingand no zinc layers are formed on such portions. However, when crevicesare formed between the steel sheet and the SiO₂, zinc layers are formedin such crevices. Consequently, a very high Zn-coating ratio may beobtained. Preferably, the coating ratio is 100%. Further, it ispreferable that zinc be deposited by a certain amount or more on thesurface of the steel sheet. Specifically, the area ratio of zincdeposited on the surface is preferably 60% or more and the area ratio ofzinc deposited in the crevices (the ratio of the portion of the crevicesdescribed above) is preferably 40% or less.

The high-strength cold rolled steel sheets manufactured by themanufacturing process have excellent conversion treatment properties. Inaddition to the aforementioned steps, the process preferably furtherincludes a P-containing aqueous solution contact step in which thehigh-strength cold rolled steel sheet is brought into contact with aphosphorus-containing aqueous solution (a P-containing aqueoussolution). The high-strength cold rolled steel sheets obtained by themanufacturing process become high-strength cold rolled steel sheets thathave undergone conversion treatment.

The conversion treatment of the high-strength cold rolled steel sheetobtained by our process generally includes an alkali degreasing step, asurface conditioning step and a zinc phosphate treatment step in thenamed order. As a result of introducing the aforementioned P-containingaqueous solution contact step, a trace amount of phosphorus is attachedto the surface of the zinc plating and, consequently, sufficientdegreasing becomes feasible even in consideration of negative factorssuch as degradation of an alkaline degreasing liquid. The mechanism ofthis effect is assumed to be as follows. In the use of a zinc sulfatebath that is a usual electrogalvanization bath, it is probable that thesulfate radicals are incorporated into the zinc plating layer andincrease the affinity for oils to make degreasing difficult. Incontrast, when the P-containing aqueous solution is brought into contactwith the steel sheet, the sulfate radicals present on the surface arewashed away and a trace amount of phosphorus is attached onto thesurface to decrease the affinity for oils. This is probably the reasonwhy degreasing properties are improved.

In the P-containing aqueous solution contact step, the P concentrationof the aqueous solution that is to be contacted with the steel sheet isnot particularly limited. The P concentration is preferably not lessthan 0.001 g/L. The treatment is particularly effective when theconcentration is 0.001 to 10 g/L. If the P concentration is less than0.001 g/L, the solution is not sufficient as it poorly washes away thesulfate radicals and the amount of phosphorus attached to the surfacebecomes insufficient. On the other hand, the upper limit is 10 g/Lbecause the effects obtained are substantially unchanged after theconcentration exceeds 10 g/L. The temperature of the P-containingaqueous solution is not particularly limited, but is preferably not lessthan 30° C. The treatment is particularly effective when the temperatureis 30 to 80° C. Sufficient effects are obtained by performing theP-containing aqueous solution contact step at a temperature of 30° C. orabove. The upper limit of the temperature is not limited from theviewpoint of effects. From the viewpoint of the temperature elevation inactual line operation, the practical upper limit is 80° C. Increasingthe temperature of the P-containing aqueous solution to above 60° C.ensures sufficient effects but is not economically efficient due toreasons such as the need of an extra heating facility. Thus, the upperlimit of the temperature is more preferably 60° C.

The P-containing aqueous solution may be brought into contact with thesteel sheet by any method without limitation. For example, a soakingmethod or a spraying method may be adopted. In a spraying method,conditions such as the spray pressure, the nozzle diameter and thedistance from the nozzle to the steel sheet are not particularly limitedas long as the aqueous solution can be brought into contact with thesteel sheet.

In the general conversion treatment, the alkali degreasing step cleansthe steel sheets of oils such as antirust oils applied to the steelsheets and press washing oils frequently used in press forming ofautomobile body exterior panels. It is sometimes difficult to removesuch oils when galvanized steel sheets are directly soaked into analkaline degreasing liquid. In particular, it is conceivable that analkaline degreasing liquid will be contaminated with oils or degradedwhen a large number of car bodies are alkali degreased continuously oneafter another on, for example, a painting line in an automobilemanufacturer. In such a case, it may be possible that oils are notremoved sufficiently and adversely affect the phosphatization treatmentin the later stage. Treatment with the P-containing aqueous solution candecrease the adverse effects on the conversion treatment even in theevent of degradation of an alkaline degreasing liquid.

For example, the alkaline degreasing liquid may be a liquid with a pH of9 to 14 that includes at least one selected from silicate salts such assodium orthosilicate, sodium metasilicate, sodium silicate No. 1 andsodium silicate No. 2, phosphate salts such as monosodium phosphate,disodium phosphate, trisodium phosphate, sodium pyrophosphate, sodiumtripolyphosphate and sodium hexametaphosphate, alkalis such as sodiumhydroxide, sodium carbonate, sodium hydrogencarbonate, sodium borate andsodium sulfite, and nonionic, anionic or cationic surfactants.

The surface conditioning step performed after the alkali degreasing stepallow a layer (a layer composed of phosphate crystal) to be depositedmore uniformly in the subsequent conversion treatment. Examples of thesurface conditioning treatments include soaking in a treatment liquidsuch as a titanium colloid-containing aqueous liquid or a zinc phosphatecolloid-containing aqueous liquid.

Thereafter, the zinc phosphate treatment step is performed. The zincphosphate treatment step is a step forming a conversion layer.

The zinc phosphate treatment may be performed by any method withoutlimitation. For example, the steel sheet may be soaked in a conversiontreatment liquid containing zinc phosphate or may be coated with such aconversion treatment liquid with a device such as a spray or a coater.

The phosphate crystals formed by the conversion treatment includephosphophyllite (Zn₂Fe(PO₄)₂.4H₂O). The phosphate crystals precipitatedalso include a large amount of hopeite (Zn₃(PO₄)₂.4H₂O). It isconventionally known that steel sheets exhibit higher post-paintingcorrosion resistance with increasing P ratio (the value of P/(P+H)obtained by the X-ray diffractometry of phosphated steel sheets whereinP is the intensity of phosphophyllite and H is the intensity ofhopeite). In recent years, however, the P ratio has no significance onthe post-painting performance due to the rapid improvements ofconversion treatment agents and electrodeposition paints.

The advantageous effects have been described hereinabove mainly in termsof the improvements in conversion treatment properties of high-Sihigh-strength cold rolled steel sheets. Improvements in post-paintingcorrosion resistance may be also obtained by virtue of the presence ofzinc on the steel sheet surface. That is, our techniques ensure bothconversion treatment properties and post-painting corrosion resistanceof cold rolled steel sheets.

The types of paints used in the painting are not particularly limitedand may be selected appropriately in accordance with purposes such asuse application. The paints may be applied by any methods withoutlimitation. Examples of the coating methods include electrodepositioncoating, roll coating, curtain flow coating and spray coating.Techniques such as hot air drying, infrared heating and inductionheating may be used to dry the paints.

The high-strength cold rolled steel sheets produced by our manufacturingprocess comprehend painted steel sheets obtained as described above.

Examples

Steels A to D having the chemical compositions described in Table 1 wereproduced by a usual steelmaking process and were continuously cast androlled into slabs. Next, the slabs were reheated to 1250° C. and werehot rolled at a finishing temperature of 850° C. and a coilingtemperature of 600° C. Thus, hot rolled sheets with a sheet thickness of3.0 mm were obtained. The hot rolled sheets were pickled and werethereafter cold rolled to a sheet thickness of 1.5 mm, thereby testsamples being prepared. With use of a laboratory reductive heatingsimulator, the test samples were heat treated in a nitrogen atmospherecontaining 10 vol % hydrogen at 800 to 850° C. In this manner, annealedsheets were produced.

The annealed steel sheets were subjected to electrolytic pickling in a100 g/L aqueous sulfuric acid solution using a stainless steel plate asthe cathode. In this process, the current density was constant at 10A/dm², and the pickling losses were varied by controlling theenergization time.

The pickled steel sheets were electroplated in an aqueous solution thatcontained 1 mol/L of zinc sulfate heptahydrate and had been adjusted topH 2.0 with sulfuric acid. As the anode, an iridium oxide plate wasused. In this manner, zinc plating was deposited on the surface. Theamounts of zinc deposited by zinc plating were varied by changing thecurrent density and the energization time. The galvanized steel sheetswere subjected to a P-containing aqueous solution contact step.

The cold rolled steel sheets produced above were analyzed with an X-raymicroanalyzer (EPMA) at an accelerating voltage of 5 kV. Based on thezinc mapping analysis results, the Zn-free area ratio (or the Zn arearatio) was calculated by image processing. Further, the followingconversion treatment was carried out to evaluate conversion treatmentproperties.

First, a bath was prepared which contained a commercial alkalinedegreasing liquid (Fine Cleaner FC-E2001 manufactured by NihonParkerizing Co., Ltd.) with a prescribed concentration. To simulatedegradation, another bath was prepared in which the degreasing liquidwas diluted to half the prescribed concentration. The steel sheets weresoaked in each of these baths for 2 minutes. The steel sheets were thenwashed with water, and the water wetting ratio was evaluated. The waterwetting ratio was rated as “◯” when the value was 80% or above, “Δ” lessthan 80%, or “×” 50% or less. The results were used as an indicator ofdegreasing properties. Degreasing properties are evaluated as “good”when the water wetting ratio is 80% or more.

Next, the cold rolled steel sheets that had been degreased with thedegreasing liquid with the prescribed concentration were soaked in asurface conditioner (PL-ZTH manufactured by Nihon Parkerizing Co., Ltd.)and thereafter phosphated by being soaked in a phosphatization liquid(PALBOND PB-L3080 manufactured by Nihon Parkerizing Co., Ltd.) at a bathtemperature of 43° C. for a treatment time of 120 seconds. Ten fields ofview of the surface of the phosphated steel sheets were observed by SEMat ×300 magnification. The steel sheets were evaluated based on thefollowing five grades (conversion grades) regarding the presence orabsence and the size of areas where phosphate crystals were not formed(non-covered areas), as well as the non-uniformity of state of crystals.Small non-covered areas were of circular shape having a diameter ofabout 10 μm.

5 Points: There were no non-covered areas, and the crystals wereuniform.4 Points: The crystals were slightly nonuniform, but there were nonon-covered areas.3 Points: There were small non-covered areas.2 Points: There were relatively large non-covered areas.1 Point: There were a large number of relatively large non-coveredareas.

Further, the steel sheets were coated with a commercial ED paint (GT-10manufactured by Kansai Paint Co., Ltd.) with a film thickness of 20 μm,and the coated surface was cross-cut with NT Cutter. The steel sheetswere then soaked in hot salt water (5% NaCl, 50° C.) for 10 days. Aftersoaking, a polyester tape was applied to cover the cross-cut areas ofthe samples and thereafter peeled therefrom. The maximum width ofpeeling on any one side of the cut lines (the width of peeling on oneside after soaking in hot salt water) was measured. The test results aredescribed in Tables 2 to 4.

TABLE 1 Chemical composition (mass %) Steel C Si Mn P S Al N A 0.10 0.501.6 0.030 0.0020 0.040 0.0036 B 0.10 1.00 2.1 0.020 0.0010 0.007 0.0040C 0.10 1.50 2.0 0.030 0.0020 0.040 0.0036 D 0.10 2.00 2.6 0.020 0.00100.007 0.0040

TABLE 2 Contact with Annealing P-containing atmosphere Heating inaqueous Dew annealing Pickling Zn-coating solution Zn area H₂ conc.point Temp. Time loss mass P conc. Temp. ratio No. Steel (vol %) (° C.)(° C.) (min) (g/m²) (mg/m²) (g/L) (° C.) (%)  1 A 10.00 −35 800 3 — — —— —  2 B ↓ ↓ 810 ↓ — — — — —  3 C ↓ ↓ 830 ↓ — — — — —  4 D ↓ ↓ 850 ↓ — —— — —  5 A ↓ ↓ 800 ↓ — 500 0.5000 50 73  6 B ↓ ↓ 810 ↓ — ↓ ↓ ↓ 75  7 C ↓↓ 830 ↓ — ↓ ↓ ↓ 75  8 D ↓ ↓ 850 ↓ — ↓ ↓ ↓ 74  9 A ↓ ↓ 800 ↓ 0.2 ↓ ↓ ↓ 7510 B ↓ ↓ 810 ↓ ↓ ↓ ↓ ↓ 73 11 C ↓ ↓ 830 ↓ ↓ ↓ ↓ ↓ 73 12 D ↓ ↓ 850 ↓ ↓ ↓ ↓↓ 73 13 A ↓ ↓ 800 ↓ 0.5 ↓ ↓ ↓ 74 14 B ↓ ↓ 810 ↓ ↓ ↓ ↓ ↓ 76 15 C ↓ ↓ 830↓ ↓ ↓ ↓ ↓ 75 16 D ↓ ↓ 850 ↓ ↓ ↓ ↓ ↓ 75 17 A ↓ ↓ 800 ↓ 1.0  50 ↓ ↓ 73 18B ↓ ↓ 810 ↓ ↓ ↓ ↓ ↓ 73 19 C ↓ ↓ 830 ↓ ↓ ↓ ↓ ↓ 74 20 D ↓ ↓ 850 ↓ ↓ ↓ ↓ ↓73 21 A ↓ ↓ 800 ↓ ↓ 150 ↓ ↓ 75 22 B ↓ ↓ 810 ↓ ↓ ↓ ↓ ↓ 75 23 C ↓ ↓ 830 ↓↓ ↓ ↓ ↓ 74 24 D ↓ ↓ 850 ↓ ↓ ↓ ↓ ↓ 73 Soaking in hot salt water Width ofAlkali degreasing peeling on Water Water Conversion one side No. Conc.wetting Conc. wetting grade (mm)  1 Prescribed ◯ 2-fold ◯ 2 6.2 Comp.Ex. 1 dilution  2 ↓ ◯ ↓ ◯ 2 7.6 Comp. Ex. 2  3 ↓ ◯ ↓ ◯ 1 8.1 Comp. Ex. 3 4 ↓ ◯ ↓ ◯ 1 8.5 Comp. Ex. 4  5 ↓ ◯ ↓ ◯ 2 5.2 Comp. Ex. 5  6 ↓ ◯ ↓ ◯ 25.5 Comp. Ex. 6  7 ↓ ◯ ↓ ◯ 1 6.0 Comp. Ex. 7  8 ↓ ◯ ↓ ◯ 1 6.3 Comp. Ex.8  9 ↓ ◯ ↓ ◯ 3 3.5 Comp. Ex. 9 10 ↓ ◯ ↓ ◯ 3 3.8 Comp. Ex. 10 11 ↓ ◯ ↓ ◯3 4.0 Comp. Ex. 11 12 ↓ ◯ ↓ ◯ 3 4.0 Comp. Ex. 12 13 ↓ ◯ ↓ ◯ 5 2.0 Inv.Ex. 1 14 ↓ ◯ ↓ ◯ 5 2.0 Inv. Ex. 2 15 ↓ ◯ ↓ ◯ 5 2.0 Inv. Ex. 3 16 ↓ ◯ ↓ ◯5 2.0 Inv. Ex. 4 17 ↓ ◯ ↓ ◯ 3 5.5 Comp. Ex. 13 18 ↓ ◯ ↓ ◯ 2 6.3 Comp.Ex. 14 19 ↓ ◯ ↓ ◯ 2 6.5 Comp. Ex. 15 20 ↓ ◯ ↓ ◯ 1 7.0 Comp. Ex. 16 21 ↓◯ ↓ ◯ 5 1.6 Inv. Ex. 5 22 ↓ ◯ ↓ ◯ 5 1.7 Inv. Ex. 6 23 ↓ ◯ ↓ ◯ 5 1.3 Inv.Ex. 7 24 ↓ ◯ ↓ ◯ 5 1.5 Inv. Ex. 8

TABLE 3 Contact with Annealing P-containing atmosphere Heating inaqueous Dew annealing Pickling Zn-coating solution Zn area H₂ conc.point Temp. Time loss mass P conc. Temp. ratio No. Steel (vol %) (° C.)(° C.) (min) (g/m²) (mg/m²) (g/L) (° C.) (%) 25 A 10.00 −35 800 3 1.01000 0.5000 50 78 26 B ↓ ↓ 810 ↓ ↓ ↓ ↓ ↓ 78 27 C ↓ ↓ 830 ↓ ↓ ↓ ↓ ↓ 77 28D ↓ ↓ 850 ↓ ↓ ↓ ↓ ↓ 77 29 A ↓ ↓ 800 ↓ ↓ 3000 ↓ ↓ 78 30 B ↓ ↓ 810 ↓ ↓ ↓ ↓↓ 79 31 C ↓ ↓ 830 ↓ ↓ ↓ ↓ ↓ 78 32 D ↓ ↓ 850 ↓ ↓ ↓ ↓ ↓ 78 33 A ↓ ↓ 800 ↓↓ 5000 ↓ ↓ 79 34 B ↓ ↓ 810 ↓ ↓ ↓ ↓ ↓ 80 35 C ↓ ↓ 830 ↓ ↓ ↓ ↓ ↓ 79 36 D ↓↓ 850 ↓ ↓ ↓ ↓ ↓ 79 37 A ↓ ↓ 800 7 ↓  500 ↓ ↓ 51 38 B ↓ ↓ 810 ↓ ↓ ↓ ↓ ↓52 39 C ↓ ↓ 830 ↓ ↓ ↓ ↓ ↓ 50 40 D ↓ ↓ 850 ↓ ↓ ↓ ↓ ↓ 50 41 A ↓ ↓ 800 31.5 ↓ ↓ ↓ 80 42 B ↓ ↓ 810 ↓ ↓ ↓ ↓ ↓ 80 43 C ↓ ↓ 830 ↓ ↓ ↓ ↓ ↓ 81 44 D ↓↓ 850 ↓ ↓ ↓ ↓ ↓ 80 Soaking in hot salt water Width of Alkali degreasingpeeling on one Water Water Conversion side No. Conc. wetting Conc.wetting grade (mm) 25 Prescribed ◯ 2-fold ◯ 5 1.4 Inv. Ex. 9 dilution 26↓ ◯ ↓ ◯ 5 1.2 Inv. Ex. 10 27 ↓ ◯ ↓ ◯ 5 1.3 Inv. Ex. 11 28 ↓ ◯ ↓ ◯ 5 1.1Inv. Ex. 12 29 ↓ ◯ ↓ ◯ 5 1.6 Inv. Ex. 13 30 ↓ ◯ ↓ ◯ 5 1.6 Inv. Ex. 14 31↓ ◯ ↓ ◯ 5 1.3 Inv. Ex. 15 32 ↓ ◯ ↓ ◯ 5 1.4 Inv. Ex. 16 33 ↓ ◯ ↓ ◯ 5 1.4Inv. Ex. 17 34 ↓ ◯ ↓ ◯ 5 1.2 Inv. Ex. 18 35 ↓ ◯ ↓ ◯ 5 1.3 Inv. Ex. 19 36↓ ◯ ↓ ◯ 5 1.7 Inv. Ex. 20 37 ↓ ◯ ↓ ◯ 3 3.8 Inv. Ex. 21 38 ↓ ◯ ↓ ◯ 3 3.5Inv. Ex. 22 39 ↓ ◯ ↓ ◯ 3 3.8 Inv. Ex. 23 40 ↓ ◯ ↓ ◯ 3 3.7 Inv. Ex. 24 41↓ ◯ ↓ ◯ 5 1.3 Inv. Ex. 25 42 ↓ ◯ ↓ ◯ 5 1.6 Inv. Ex. 26 43 ↓ ◯ ↓ ◯ 5 1.6Inv. Ex. 27 44 ↓ ◯ ↓ ◯ 5 1.2 Inv. Ex. 28

TABLE 4 Contact with Annealing P-containing atmosphere Heating in Zn-aqueous Dew annealing Pickling coating solution Zn area H₂ conc. pointTemp. Time loss mass P conc. Temp. ratio No. Steel (vol %) (° C.) (° C.)(min) (g/m²) (mg/m²) (g/L) (° C.) (%) 45 A 1.00 −35° C. 800 3 1.0 5000.5000 50 79 46 B ↓ ↓ 810 ↓ ↓ ↓ ↓ ↓ 78 47 C ↓ ↓ 830 ↓ ↓ ↓ ↓ ↓ 78 48 D ↓↓ 850 ↓ ↓ ↓ ↓ ↓ 79 49 A 0.10 ↓ 800 ↓ ↓ ↓ ↓ ↓ 80 50 B ↓ ↓ 810 ↓ ↓ ↓ ↓ ↓80 51 C ↓ ↓ 830 ↓ ↓ ↓ ↓ ↓ 79 52 D ↓ ↓ 850 ↓ ↓ ↓ ↓ ↓ 80 53 C 10.00  ↓ 830↓ ↓ ↓ — — 75 54 C ↓ ↓ ↓ ↓ ↓ ↓ 0.0005 50° C. 75 55 C ↓ ↓ ↓ ↓ ↓ ↓ 0.001050° C. 75 56 C ↓ ↓ ↓ ↓ ↓ ↓ 0.1000 50° C. 75 57 C ↓ ↓ ↓ ↓ ↓ ↓ 2.0000 50°C. 75 58 C ↓ ↓ ↓ ↓ ↓ ↓ 10.000 50° C. 75 59 C ↓ ↓ ↓ ↓ ↓ ↓ 0.5000 20° C.75 60 C ↓ ↓ ↓ ↓ ↓ ↓ 0.5000 30° C. 75 61 C ↓ ↓ ↓ ↓ ↓ ↓ 0.5000 60° C. 7562 C ↓ ↓ ↓ ↓ ↓ ↓ 0.5000 70° C. 75 63 C ↓ ↓ ↓ ↓ ↓ ↓ 2.0000 70° C. 75 64 C↓ ↓ ↓ ↓ ↓ ↓ 10.000 70° C. 75 Soaking in hot salt water Width of Alkalidegreasing peeling on Water Water Conversion one side No. Conc. wettingConc. wetting grade (mm) 45 Prescribed ◯ 2-fold ◯ 5 1.1 Inv. Ex. 29dilution 46 ↓ ◯ ↓ ◯ 5 1.2 Inv. Ex. 30 47 ↓ ◯ ↓ ◯ 5 1.2 Inv. Ex. 31 48 ↓◯ ↓ ◯ 5 1.2 Inv. Ex. 32 49 ↓ ◯ ↓ ◯ 5 1.2 Inv. Ex. 33 50 ↓ ◯ ↓ ◯ 5 1.0Inv. Ex. 34 51 ↓ ◯ ↓ ◯ 5 1.1 Inv. Ex. 35 52 ↓ ◯ ↓ ◯ 5 0.8 Inv. Ex. 36 53↓ ◯ ↓ X 5 1.3 Inv. Ex. 37 54 ↓ ◯ ↓ Δ 5 1.6 Inv. Ex. 38 55 ↓ ◯ ↓ ◯ 5 1.6Inv. Ex. 39 56 ↓ ◯ ↓ ◯ 5 1.2 Inv. Ex. 40 57 ↓ ◯ ↓ ◯ 5 1.1 Inv. Ex. 41 58↓ ◯ ↓ ◯ 5 1.0 Inv. Ex. 42 59 ↓ ◯ ↓ Δ 5 1.2 Inv. Ex. 43 60 ↓ ◯ ↓ ◯ 5 1.3Inv. Ex. 44 61 ↓ ◯ ↓ ◯ 5 1.5 Inv. Ex. 45 62 ↓ ◯ ↓ ◯ 5 1.1 Inv. Ex. 46 63↓ ◯ ↓ ◯ 5 1.2 Inv. Ex. 47 64 ↓ ◯ ↓ ◯ 5 1.2 Inv. Ex. 48

From Table 1, all the steel sheets contained a large amount of silicon.From Table 2, those steel sheets that had been phosphated afterannealing without the annealing pickling step and the electroplatingstep (Comparative Examples 1 to 4) were evaluated to have a largeproportion of non-covered areas in the conversion layer. Further, thepeeling width after soaking of the painted steel sheets in hot saltwater resulted in large values.

Substantially no improvements were obtained in the judgement of thestate of conversion layers or in the width of peeling after soaking inhot salt water in the example in which zinc was deposited to ourannealed steel sheet in our specified coating mass, but the steel sheethad not been pickled in accordance with our procedures (ComparativeExamples 5 to 8) or in the example in which the steel sheet was pickled,but the pickling loss was outside our specified range (ComparativeExamples 9 to 12).

In contrast, we found that a conversion layer was formed uniformlywithout the occurrence of non-covered areas and the width of peelingafter soaking in hot salt water was stably small in the example in whichthe annealed steel sheet was pickled and galvanized with a coating massin our specified amount (Inventive Examples 1 to 4).

On the other hand, little improvement was obtained in the state ofconversion layers and the width of peeling after soaking in hot saltwater was large in the example in which the annealed steel sheet waspickled to a sufficient extent, but the zinc-coating mass was below ourspecified range (Comparative Examples 13 to 16).

We found that all the characteristics were satisfied to a sufficientextent in the example in which the pickling loss and the zinc-coatingmass were within our ranges (Inventive Examples 5 to 20).

In Inventive Examples 21 to 24, the surface conditions were varied byextending the heating time in the annealing over the usual length oftime. As compared to other Inventive Examples, the conversion gradeswere relatively low and the widths of peeling after soaking in hot saltwater were large in spite of the fact that the pickling loss and thezinc-coating mass were within our specified ranges. This shows that notonly the coating mass, but the ratio of the areas in which zinc isdistributed on the surface should be taken into consideration.

Improvements in the quality of conversion layers and in the widths ofpeeling after soaking in hot salt water were obtained also when thepickling loss was varied (Inventive Examples 25 to 28) or when thehydrogen concentration during the annealing was varied (InventiveExamples 29 to 36).

The electroplated steel sheet which had not been contacted with aP-containing aqueous solution (Inventive Example 37) or which had beencontacted with a low concentration of phosphorus (Inventive Example 38)exhibited sufficient degreasing properties when the degreasing liquidhad the prescribed concentration. However, these steel sheets repelledwater after the degreasing treatment when the degreasing liquid had thediluted concentration which simulated degradation in an actual paintingline. Similarly, the steel sheet failed to exhibit sufficient degreasingproperty in the treatment with the diluted degreasing liquid when thesteel sheet had been contacted with a high concentration of phosphorus,but the temperature of the treatment liquid was low (Inventive Example43). In contrast, the steel sheets achieved sufficient degreasingproperties even with respect to the diluted degreasing liquid when the Pconcentration and the temperature of the treatment liquid were withinour ranges (Inventive Examples 39 to 42, and 44 to 48).

INDUSTRIAL APPLICABILITY

Even high-strength cold rolled steel sheets containing large amounts ofalloying elements achieve good conversion treatment properties beforepainting and exhibit good corrosion resistance after painting, and thusmay be used in automobile body applications.

1.-3. (canceled)
 4. A process of manufacturing high-strength cold rolledsteel sheets containing 0.5 to 2.0 mass % silicon, comprising: apickling step of thermally annealing a steel sheet in a non-oxidizingatmosphere and thereafter pickling the steel sheet to dissolve away 0.5g/m² to less than 2.0 g/m² of the steel sheet, and an electroplatingstep of electroplating the surface of the pickled steel sheet with zincunder such conditions that a coating mass of 100 to 5000 mg/m² isobtained.
 5. The process according to claim 4, wherein the non-oxidizingatmosphere is obtained by introducing a mixture gas containing nitrogenand hydrogen, the hydrogen content in the non-oxidizing atmosphere isnot more than 10 vol %, and the temperature of heating during thethermal annealing is not more than 900° C.
 6. The process according toclaim 4, further comprising an aqueous solution contact step of bringingthe steel sheet after the electroplating step into contact with aP-containing aqueous solution having a concentration of not less than0.001 g/L at a temperature of the P-containing aqueous solution of notless than 30° C.
 7. The process according to claim 5, further comprisingan aqueous solution contact step of bringing the steel sheet after theelectroplating step into contact with a P-containing aqueous solutionhaving a concentration of not less than 0.001 g/L at a temperature ofthe P-containing aqueous solution of not less than 30° C.